Substrate for information recording medium and magnetic recording medium composed of crystallized glass

ABSTRACT

Provided is a substrate for information recording medium composed of such a crystallized glass as having high Young&#39;s modulus, strength and heat resistance, being excellent in surface smoothness, surface homogeneity and surface processability, as well as having a relatively low temperature of glass liquid phase and being capable of producing cheaply, and an information recording medium using this substrate.  
     The crystallized glass substrate for information recording medium comprising 35-65 mol % of SiO 2 , 5-25 mol % of Al 2 O 3 , 10-40 mol % of MgO and 5-15 mol % of TiO 2 , in which the total amount of aforementioned composition is at least equal to or higher than 92 mol % and the main crystals are enstatite and/or its solid solution. The information recording medium having this substrate and a recording layer formed on said substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/612,015, filed Jul. 3, 2003, which in turn is a divisional of U.S.patent application Ser. No. 09/610,687, filed Jul. 7, 2000, now U.S.Pat. No. 6,627,566, claiming priority of Japanese Patent Application No.193574/1999, filed Jul. 7, 1999, and Japanese Patent Application No.293003/1999, filed Oct. 14, 1999.

FIELD OF THE INVENTION

This invention relates to crystallized glasses suitable for substrateswhich are used for information recording media, such as magnetic disks,optical disks and optical magnetic disks, substrates for informationrecording media composed of this crystallized glass, and informationrecording media using said substrate for information recording medium.

DESCRIPTION OF THE RELATED ART

Major components of magnetic storage devices of electronic computers andthe like are a magnetic recording medium and a magnetic head forreconstruction of magnetically recorded information. Flexible disks andhard disks have been known as magnetic recording media. As substratesfor hard disks, aluminum alloy has been mainly used. Recently, flyingheight of magnetic heads is markedly reduced as hard disk drivers fornotebook personal computers are made smaller and their magneticrecording density made higher. Accordingly, extremely high precision hasbeen demanded for the surface smoothness of magnetic disk substrates.However, it is difficult to produce smooth surface more than a certainlevel of precision with an aluminum alloy. That is, even though it ispolished using highly precise abrasives and processing apparatuses, thepolished surface may suffer from plastic deformation because of the lowhardness of the alloy. Even if the aluminum alloy is plated withnickel-phosphorus, the surface roughness Ra cannot be made equal to orless than 5 Å (angstrom). In addition, as hard disk drivers are madesmaller and thinner, a further smaller thickness of substrates formagnetic disks is also strongly desired. However, it is difficult toproduce such a thin disk with an aluminum alloy having a certainstrength defined by specification of hard disk drivers because of lowstrength and stiffness of aluminum alloy.

Therefore, glass substrates for magnetic disks of which high strength,high stiffness, high impact resistance and high surface smoothness arerequired have been developed. Among these, chemically reinforced glasssubstrates whose surfaces are strengthened by the ion exchangetechnique, crystallized glass substrates subjected to crystallizationtreatment and the like have been known well.

As a chemically reinforced glass substrate by ion-exchange, for example,a glass disclosed in Japanese Patent Unexamined Publication No. Hei.1-239036 (JP-A-239036/89) has been known. This chemically reinforcedglass substrate is such a glass substrate for magnetic disks as theglass containing, indicated in terms of % by weight, 50-65% of SiO₂,0.5-14% of Al₂O₃, 10-32% of R₂O where R is an alkali metal ion, 1-15% ofZnO and 1.1-14% of B₂O₃ is reinforced by forming a crushing stress layeron the glass substrate with an ion exchange method by an alkali ion.

In addition, as a crystallized glass, for example, a glass disclosed inJapanese Patent Examined Publication No.2516553 is exemplified. Thiscrystallized glass is such a crystallized glass for magnetic disks whichcontains, indicated in terms of % by weight, 65-83% of SiO₂, 8-13% byLi₂O, 0-7% of K₂O, 0.5-5.5% of MgO, 0-5% of ZnO, 0-5% of PbO (providedthat MgO+ZnO+PbO is 0.5-5%), 1-4% of P₂O₅, 0-7% of Al₂O₃ and 0-2% ofAs₂O₃+Sb₂O₃, and contains micro crystalline particles of Li₂O.2SiO₂asmain crystals.

Moreover, a crystallized glass is also disclosed in Japanese PatentUnexamined Publication No. Hei.7-291660 (JP-A-291660/95). Thiscrystallized glass is obtained by heat treatment after fusion andforming a glass which consists of, indicated in terms of % by weight,38-50% of SiO₂, 18-30% of Al₂O₃, 10-20% of MgO, provided that having acomposition containing, indicated in terms of weight ratio, 1.2-2.3 ofAl₂O₃/MgO, 0%-5% of B₂O₃, 0%-5% of CaO, 0%-5% of BaO, 0%-5% of SrO,0.5%-7 .5% of ZnO, 4%-15% of TiO₂, 0%-5% of ZrO₂ and 0%-2% of As₂O₃and/or Sb₂O₃. This glass is a cordierite based crystallized glasscharacterized by containing cordierite based crystals as crystals.Moreover, a substrate for magnetic disks composed of this crystallizedglass is also disclosed.

In addition, a crystallized glass is also disclosed in Japanese PatentUnexamined Publication No. Hei.9-77531 (JP-A-77531/97) (U.S. Pat. No.5,476,821). This crystallized glass is a ceramic product having theYoung's modulus in the range of from about 14×10⁶ about 24×10⁶ psi(96-165 GPa) and the fracture toughness of more than 10 MPa.m^(1/2). Inaddition, this crystallized glass consists of crystalline phaselaminated body which mainly consists of crystals having a spinelstructure and a uniform size and dispersing uniformly in asiliceous-rich residual matrix. This is a glass ceramic whichsubstantially consists of, indicated in terms of % by weight usingoxides as a standard, 35-60% of SiO₂, 20-35% of Al₂O₃, 0-25% of MgO,0-25% of ZnO, 0-20% of TiO₂, 0-10% of ZrO₂, 0-2% of Li₂O and 0-8% ofNiO. This has at least about 10% of a total amount of MgO+ZnO and maycontain equal to or less than 5% of an optional component selected fromthe group composed of BaO, CaO, PbO, SrO, P₂O₅, B₂O₃ and Ga₂O₃, in therange. of from 0 to 5% of R₂O selected from the group consisting ofNa₂O, K₂O, Rb₂O and Cs₂O, and in the range of f rom 0 to 8% oftransition metals. In the case of containing less than about 25% ofAl₂O₃, this is a glass ceramic having a composition in which the totalamount of TiO₂+ZrO₂+NiO is equal to or more than 5%. In abovepublication, the substrate for magnetic disks consisting of this glassceramic is disclosed.

In addition, a crystallized glass is also disclosed in U.S. Pat. No.5,491,116. This crystallized glass is a glass ceramic product having thefracture coefficient of at least about 15,000 psi, the Knoop hardnessexceeding about 760 KHN, the Young's modulus of more than about 20×10⁶psi and the fracture toughness of more than 1.0 MPa.m^(1/2). The maincrystals of the crystallized glass are enstatite or its solid solutionand spinel (spinel structure crystal), and the crystallized glasscontains at least 92% of the composition substantially composed of,indicated in terms of % by weight, 35-60% of SiO₂, 10-30% of Al₂O₃,12-30% of MgO, 0-10% of ZnO, 5-20% of TiO₂and 0-8% of Nio. Moreover, thesubstrate for magnetic disks composed of this crystallized glass is alsodisclosed. It is to be noted that the same glass as the crystallizedglass disclosed in aforementioned patent is also disclosed in Journal ofNon-Crystalline Solids 219(1997) 219-227.

However, along with making hard disks smaller and thinner and makingrecording density higher, it is rapidly developed to make flight heightof magnetic heads smaller and revolution speed of disks higher. Thereby,substrate materials are more strictly required the strength, the Young'smodulus, the smoothness of the surface and the like. In particular, bymaking the information recording density of 3.5-inch hard disks forpersonal computers and severs higher, the surface smoothness and thesurface flatness of the substrate materials are strictly required. Inaddition, corresponding to the higher data processing speed, it isrequired to set the winding number of the disks equal to or higher than10,000 rpm. Thus, the requirement for stiffness of substrate materialsbecomes increasingly severer, and the limitation of conventionalaluminum substrates already becomes obvious. In future, as long as it isnecessarily demanded to make the capacity of hard disks higher and tomake the revolution speed of hard disks higher, it is clear that thesubstrate materials for magnetic recording medium is strongly requiredto exhibit higher Young's modulus, higher strength, more excellentsurface flatness, higher impact resistance and the like.

However, such a chemically reinforced glass as disclosed in JapanesePatent Unexamined Publication No. Hei. 1-239036 (JP-A-239036/89)mentioned above has the Young's modulus of about 80 GPa, therefore, itcannot meet the strict demand for hard disks in future. As forconventional chemically reinforced substrate glasses, alkali ions areintroduced in a large amount in the glass for ion exchange, so that thereinforced glasses mostly have the low Young's modulus (90 GPa).Moreover, due to also having low stiffness, it cannot meet 3.5-inchhigh-end disk substrates and thinner disk substrates. In addition, theglass chemically reinforced by ion exchange contains large amount ofalkali components. Thus, if it is used for long hours under thecircumstance of high temperature and high humidity, alkali ions depositfrom parts including thin magnetic films or exposing glasses, such as apinhole part of a magnetic film or a circumference of a magnetic film.It has a disadvantage that this triggers a corrosion or decomposition ofthe magnetic films. In the producing process of the magnetic recordingmedium, after providing a magnetic layer on the glass substrate, certainheat treatment may be carried out in order to improve characteristicssuch as coercive force of the magnetic layer. However, the conventionalion-exchanged reinforced glass mentioned above has at most 500° C. ofthe glass transition temperature, so that it has poor heat resistance.Thereby, it has a problem that higher coercive force cannot be obtained.

In addition, the conventional crystallized glass as disclosed inJapanese Patent Publication No. 2516553 mentioned above is superior alittle in the Young's modulus and heat resistance than aforementionedchemically reinforced glass substrate. However, the surface roughness isequal to or higher than 10 Å, thereby the surface smoothness is poor, sothat it is limited to make the flying height smaller. Therefore, it hasa problem that it cannot meet higher magnetic recording density.Moreover, the Young's modulus is about from 90 to 100 GPa at most, sothat it also cannot meet 3.5-inch high-end disk substrates and thinnerdisk substrates.

In addition, the crystallized glass disclosed in Japanese PatentUnexamined Publication No. Hei.7-291660 (JP-A-291660/95) mentioned abovehas the Young's modulus of about from 100 to 130 GPa, therefore, itcannot be said that it is sufficient. Moreover, it has only such asurface smoothness as having the Young's modulus of about 8 Å, resultingin poor smoothness. Additionally, the temperature of glass liquid phaseis high which is 1400° C., so that it has a disadvantage of difficultyin producing.

Moreover, the crystallized glass disclosed in Japanese Patent UnexaminedPublication No. Hei. 9-77531 (JP-A-77531/97) mentioned above has adisadvantage of large difficulty in polishing because the main crystalsare spinel.

Moreover, a large amount of enstatite is contained together with spinelin the crystallized glass disclosed in U.S. Pat. No. 5,491,116 andJournal of Non-Crystalline Solids 219(1997) 219-227. Accordingly, it canbe considered that easiness in polishing is more improved than thecrystallized glass disclosed in Japanese Patent Unexamined PublicationNo. Hei.9-77531 (JP-A-77531/97). However, because spinel is stillcontained therein, it is difficult to say that it has sufficientpolishing characteristics. That is, it still takes a long time forpolishing required for obtaining desirable surface roughness, so that ithas a problem of inferior productivity.

Moreover, because a glass disclosed in Japanese Patent Publication No.2648673 is a fire-resistant glass ceramic for the purpose of usingtemperature equal to or higher than 1200° C., it is difficult to be usedas a substrate for information recording medium. That is, it isdifficult to be produced due to high melting temperature, moreover, thesurface smoothness required for information recording medium cannot beobtained due to large crystal size.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide asubstrate for information recording medium, such as magnetic disks andthe like, consisting of crystallized glass which has higher Young'smodulus, higher strength and higher fire-resistance, superior in thesurface smoothness, surface homogeneity and surface processability withconsidering demands that a substrate for magnetic recording medium willbe made thinner and will have high strength, high heat resistance, highimpact resistance and the like in near future, as well as it can becheaply produced due to comparatively low temperature of the glassliquid phase.

Moreover, it is also an object of the present invention to provideinformation recording media using the substrate composed ofaforementioned crystallized glass, such as magnetic disks.

In addition, the present invention provides a production method of thesubstrate for information recording medium composed of aforementionedcrystallized glass.

To solve objects mentioned above, the inventors carried out variousexaminations, as results, it is found that crystallized glass suitablefor substrate for information recording medium which has high Young'smodulus equal to or higher than 140 GPa, good surface smoothness, andcomparatively low liquid temperature. Then, they accomplished thepresent invention.

The present invention relates to a substrate for information recordingmedium (hereinafter referred to substrate 1) composed of crystallizedglass comprising

-   -   SiO₂: 35-65 mol %    -   Al₂O₃: 5-25 mol %    -   MgO: 10-40 mol % and    -   TiO₂: 5-15 mol %,    -   wherein the sum of the above components is equal to or more than        92 mol %, andmain crystals contained in the crystallized glass        are enstatite and/or its solid solution.

With the substrate above, a molar ratio of Al₂O₃ to MgO (Al₂O₃/MgO) maybe from equal to or more than 0.2 to less than 0.5.

With the substrate above, the crystallized glass preferably comprises

-   -   SiO₂: 40-60 mol %    -   Al₂O₃: 7-22 mol %    -   MgO: 12-35 mol % and    -   TiO₂ : 5.5-14 mol %.

With the above substrate 1, the crystallized glass may comprise Y₂O₃ inan amount equal to or less than 10 mol %.

With the above substrate 1, the crystallized glass may comprise ZrO₂ inan amount equal to or less than 10 mol %.

The present invention further relates to a substrate for informationrecording medium (hereinafter referred to substrate 2) composed ofcrystallized glass consisting essentially of

-   -   SiO₂: 35-65 mol %    -   Al₂O₃: 5-25 mol %    -   MgO: 10-40 mol %    -   TiO₂: 5-15 mol %    -   Y₂O₃: 0-10 mol %    -   ZrO₂: 0-10 mol %    -   R₂O : 0-5 mol % (wherein R is at least one selected from the        group of Li, Na and K)    -   RO: 0-5 mol % (wherein R is at least one selected from the group        of Ca, Sr and Ba)    -   As₂O₃+Sb₂O₃: 0-2 mol %    -   SiO₂+Al₂O₃+MgO+TiO₂: 92 mol % or more; and main crystals        contained in the crystallized glass are enstatite and/or its        solid solution.

The present invention further relates to a substrate for informationrecording medium (hereinafter referred to substrate 3) composed ofcrystallized glass consisting essentially of

-   -   SiO_(2 : 35)-65 mol %    -   Al₂O₃ : 5-25 mol %    -   MgO: 10-40 mol %    -   TiO₂: 5-15 mol %    -   Y₂O₃: 0-10 mol %    -   ZrO₂: 0-10 mol %    -   R₂O: 0-5 mol % (wherein R is at least one selected from the        group of Li, Na and K)    -   RO: 0-5 mol % (wherein R is at least one selected from the group        of Ca, Sr and Ba)    -   As₂O₃+Sb₂O₃: 0-2 mol %    -   SiO₂+Al₂O₃+MgO+TiO₂: 92 mol % or more;        and the crystallization degree of the crystallized glass is in a        range of 20 to 70 vol %.

With the above substrates 1-3, the crystallized glass may comprise Y₂O₃in an amount of 0.3 to 8 mol %.

With the above substrates 1-3, the crystallized glass may comprise ZrO₂in an amount of 1 to 10 mol %.

With the above substrates 1-3, the crystallized glass may comprise ZrO₂in an amount of 1 to 5 mol %.

With the above substrate 1, the crystallized glass may comprise R₂O inan amount of 1 to 5 mol %, wherein R is at least one selected from thegroup of Li, Na and K.

The R₂O is preferably K₂O.

With the above substrates 1-3, the crystallized glass may comprise TiO₂in an amount of 8 to 14 mol %.

With the above substrates 1-3, the substrate may exhibit a Young modulusequal to or more than 140 Gpa.

With the above substrate 1, the crystallized glass may comprise

-   -   SiO₂: 35-43 mol %    -   Al₂O₃: 9-20 mol %    -   MgO: 30-39 mol %,    -   Y₂O₃: 1-3 mol %    -   TiO₂: 8.5-15 mol %, and    -   ZrO₂: 1-5 mol %.

With this substrate, a molar ratio of Al₂O₃ to MgO (Al₂O₃/MgO) may beequal to or more than 1.35, and the substrate may exhibit a Youngmodulus equal to or more than 160 GPa.

With the above substrates 1-3, the mean particle size of the crystalparticles contained in the crystallized glass may be equal to or lessthan 100 nm.

With the above substrates 1-3, the mean particle size of the crystalparticles contained in the crystallized glass is equal to or less than70 nm.

With the above substrates 1-3, the substrate may have a polished surfacewith a surface roughness Ra (JIS B0601) equal to or less than 1 nm.

The present invention further relates to a substrate for informationrecording medium (hereinafter referred to substrate 4) composed ofcrystallized glass comprising enstatite and/or its solid solution asmain crystals and the substrate has a polished surface with a surfaceroughness Ra (JIS B0601) equal to or less than 1 nm.

With this substrate 4, the substrate may have a polished surface with asurface roughness Ra (JIS B0601) equal to or less than 0.5 nm.

With the above substrates 1-4, light transparency at 600 nm through thesubstrate with 1 mm thickness may be equal to or more than 10%.

With the above substrates 1-4, thermal extension coefficient of thecrystallized glass may in the range of from 65×10⁻⁷ to 85×10⁻⁷/° C.

The present invention further relates to a substrate for informationrecording medium (hereinafter referred to substrate 5) composed ofcrystallized glass comprising enstatite and/or its solid solution asmain crystals and the mean particle size of the crystal particlescontained in the crystallized glass as main crystals is equal to or lessthan 100 nm.

With this substrate 5, the mean particle size of the crystal particlescontained in the crystallized glass as main crystals ay be equal to orless than 70 nm

The present invention further relates to a substrate for informationrecording medium (hereinafter referred to substrate 6) composed ofcrystallized glass comprising enstatite and/or its solid solution asmain crystals and light transparency at 600 nm through the substratewith 1 mm thickness is equal to or more than 10%.

With the above substrates 1-6, the crystallization degree of thecrystallized glass is equal to or more than 50 vol %.

With the above substrates 1-6, the total content of enstatite and/or itssolid solution may range from 70 to 90 vol %, the content of titanatemay range from 10 to 30 vol %, and the sum of enstatite and/or its solidsolution and titanate may be equal to or more than 90 vol %.

The present invention relates to a substrate for information recordingmedium (hereinafter referred to substrate 7) composed of crystallizedglass comprising enstatite and/or its solid solution as main crystalsand thermal extension coefficient of the crystallized glass is in therange of from 65×10⁻⁷ to 85×10⁻⁷/° C.

With this substrate 1-7, the thermal extension coefficient of thecrystallized glass is in the range of from 73×10⁻⁷ to 83×10⁻⁷/° C.

With the above substrates 1-7, the crystallized glass substantially doesnot comprise quarts solid solution as the main crystals.

With the above substrates 1-7, the crystallized glass substantially doesnot comprise spinel as a crystalline phase.

With the above substrates 1-7, the crystallized glass substantially maynot comprise ZnO.

With the above substrates 1-7, the information recording medium may be amagnetic disk.

The present invention relates to an information recording mediumcomprising a recording layer on the above-mentioned substrate of thepresent invention.

With the above information recording medium, the recording layer may bea magnetic recording layer.

The present invention relates to a process for preparation of asubstrate for an information recording medium composed of crystallizedglass (hereinafter referred to process 1) comprising

-   -   SiO₂: 35-65 mol %    -   Al₂O₃: 5-25 mol %    -   MgO: 10-40 mol % and    -   TiO₂: 5-15 mol %,    -   wherein the sum of the above components is equal to or more than        92mol %, andmain crystals contained in the crystallized glass        are enstatite and/or its solid solution;    -   wherein the above process comprises steps of:    -   melting glass starting materials at 1400 to 1650° C. to prepare        a glass, molding the resulting glass into a plate-shaped glass,        and subjecting the plate-shaped glass to crystallization.

With the above process 1, the glass starting materials may comprise K₂Oand the melting temperature is from 1450 to 1600° C., preferably from1450 to 1550° C.

With the above process 1, the glass starting materials may comprise Y₂O₃and the molding of the glass into a plate shape is conducted with a moldat a temperature of from 600 to 680° C.

The present invention relates to a process for preparation of asubstrate for an information recording medium composed of crystallizedglass (hereinafter referred to process 2) comprising

-   -   SiO₂: 35-65 mol %    -   Al₂O₃: 5-25 mol %    -   MgO: 10-40 mol % and    -   TiO₂: 5-15 mol %,    -   Y₂O₃: 0-10 mol %    -   ZrO₂: 0-10 mol %    -   R₂O: 0-5 mol % (wherein R is at least one selected from the        group of Li, Na and K)    -   RO: 0-5 mol % (wherein R is at least one selected from the group        of Ca, Sr and Ba)    -   As₂O₃+Sb₂O₃: 0-2 mol %    -   SiO₂+Al₂O₃+MgO+TiO₂: 92mol % or more;    -   and main crystals contained in the crystallized glass are        enstatite and/or its solid solution;    -   wherein the above process comprises steps of:    -   melting glass starting materials at 1400 to 1650° C. to prepare        a glass,    -   molding the resulting glass into a plate-shaped glass, and        subjecting the plate-shaped glass to crystallization.

With the above processes 1-2, the crystallization may be carried out byheating the molded glass to a temperature of from 850 to 1150° C.

With the above processes 1-2, the heating maybe carried out by heatingthe molded glass to a temperature of from 500 to 850° C. at a heatingrate of 5 to 50° C./min and then heating the molded glass at a heatingrate of 0.1 to 10° C./min.

The present invention further relates to a substrate for an informationrecording medium composed of crystallized glass (hereinafter referred tosubstrate 8) comprising

-   -   SiO₂: 35-65 mol %    -   Al₂O₃: 5-25 mol %    -   MgO: 10-40 mol % and    -   TiO₂: 5-15 mol %,    -   wherein the sum of the above components is equal to or more than        92 mol %, main crystals contained in the crystallized glass are        enstatite and/or its solid solution, and the crystal glass does        not comprise ZnO;    -   wherein the above crystallized glass is prepared by a process        comprising a step of heat-treatment of a glass comprising SiO₂,        Al₂O₃, MgO and TiO₂ at a temperature of from 850 to 1150° C. to        obtain a crystallized glass.

The present invention further relates to a substrate for an informationrecording medium (hereinafter referred to substrate 9) composed ofcrystallized glass substantially consisting of

-   -   SiO₂: 35-65 mol %    -   Al₂O₃: 5-25 mol %    -   MgO: 10-4.0 mol % and    -   TiO₂: 5-15 mol %,    -   Y₂O₃: 0-10 mol %    -   ZrO₂: 0-10 mol %    -   R₂O: 0-5 mol % (wherein R is at least one selected from the        group of Li, Na and K)    -   RO: 0-5 mol % (wherein R is at least one selected from the group        of Ca, Sr and Ba)    -   As₂O₃+Sb₂O₃: 0-2 mol %    -   SiO₂+Al₂O₃+MgO+TiO₂: 92 mol % or more;    -   wherein the above crystallized glass is prepared by a process        comprising a step of heat-treatment of a glass comprising SiO₂,        Al₂O₃, MgO and TiO₂ at a temperature of from 850 to 1150° C. to        obtain a crystallized glass.

With the above substrates 8-9, the heat treatment may be carried out for1 to 4 hours.

With the above substrates 8-9, the heat-treatment may be carried out ata temperature of from 875 to 1000° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic sectional view of a magnetic disk 1 of thepresent invention which comprises crystallized glass substrate 2, onwhich unevenness control layer 3, underlying layer 4, magnetic layer 5,protective layer 6 and lubricating layer 7 are provided in this order.

MODE FOR CARRYING OUT OF THE INVENTION

[Crystallized Glass]

The substrate for information recording medium of the present inventionconsists of such a crystallized glass as containing 35-65 mol % of SiO₂,5-25 mol % of Al₂O₃, 10-40 mol % of MgO and 5-15 mol % of TiO₂, whereinthe sum of aforementioned composition is at least equal to or higherthan 92 mol %, as well as the main crystals are enstatite and/or itssolid solution.

Each of the components in the crystallized glass constituting thesubstrate of the present invention will be explained in the followings.Provided that “%” means “mol %”, as long as it is not especiallymentioned.

SiO₂ is a component of the network structure of glass, in addition, itis also a component of enstatite having a composition of MgO.SiO₂ andenstatite solid solution having a composition of (Mg.Al)SiO₃ which arethe main deposited crystals. Because the melted glass is very unstablein the case of having the content of SiO₂ of less than 35%, it is afraidthat it cannot be molded at high temperature, in addition, crystals asmentioned above become difficult to deposit. If the content of SiO₂ isless than 35%, the chemical durability and heat resistance of residualglass matrix phase tend to deteriorate. On the other hand, if thecontent of SiO₂ exceeds 65%, enstatite becomes difficult to deposit asmain crystals and the Young's modulus of glass tends to be loweredrapidly. Therefore, the content of SiO₂ranges from 35 to 65% in view ofkinds of the deposited crystals, the deposited amount thereof, chemicaldurability, heat resistance, molding characteristics and productivity.From the viewpoint that such a crystallized glass can be obtained ashaving more preferable physical characteristics, the content of SiO₂preferably ranges from 40 to 60%.

It is to be noted that, as described below, the surface smoothness maybecome inferior a little, however, it may be preferable that the contentof SiO₂ ranges from 35 to 43% because a crystallized glass having thehigh Young's modulus equal to or higher than 160 GPa can be obtained bycombination with other components.

Al₂O₃ is a medium oxide of glass and contributes to improvement of glasssurface hardness. However, if the glass content is less than 5%,chemical durability of glass matrix phase is lowered and the strengthrequired to substrate materials tends to be difficult to be obtained. Onthe other hand, if the content of Al₂O₃ exceeds 25%, enstatite as a maincomponent becomes difficult to deposit as well as the glass becomedifficult to melt due to the high melting temperature. In addition, theglass has a tendency to be easily delitrified and to increase moldingdifficulty. Accordingly, it is appropriate that the content of Al₂O₃ranges from 5 to 25%, preferably from 7 to 22% in view of meltingcharacteristics, molding characteristics at a high temperature, kinds ofdeposited crystals of glass and the like.

It is to be noted that, as described below, the surface smoothness maybecome inferior a little, however, it may be preferable that the contentof Al₂O₃ ranges from 9 to 20% because a crystallized glass having thehigh Young's modulus equal to or higher than 160 GPa can be obtained bycombination with other components.

MgO is a glass modifying component, in addition, a main component ofcrystals of enstatite having a composition of MgO.SiO₂ and its solidsolution. If the content of MgO is less than 10%, crystals as mentionedabove are difficult to deposit, having a high devitrifying tendency anda high melting temperature of the glass, as well as the workingtemperature width of glass molding tends to be narrower. On the otherhand, if the content of MgO exceeds 40%, viscosity of glass at a hightemperature is rapidly lowered, thereby, it becomes thermally unstable,productivity deteriorates and the Young's modulus and durability tend tobe lowered. Then, it is appropriate that the content of MgO ranges from10 to 40%, preferably from 12 to 35% in view of productivity, chemicaldurability, high temperature viscosity and strength of glass.

It is to be noted that, as described below, the surface smoothness maybecome inferior a little, however, it may be preferable that the contentof MgO ranges from 30 to 39% because a crystallized glass having thehigh Young's modulus equal to or higher than 160 GPa can be obtained bycombination with other components.

Provided that the contents of MgO and Al₂O₃ are adjusted so that themolar ratio of Al₂O₃ to MgO(Al₂O₃/MgO) is less than 0.5. Because theYoung's modulus of the crystallized glass rapidly lowers if the molarratio of Al₂O₃ to MgO (Al₂O₃/MgO) becomes equal to or higher than 0.5.By adjusting the Al₂O₃/MgO less than 0.5, such a crystallized glass canbe also obtained as having high Young's modulus equal to or higher than150 GPa. Al₂O₃/MgO is preferably less than 0.45. However, if the molarratio of Al₂O₃/MgO is adjusted to excessively low, it is afraid thatviscosity of glass at a high temperature tends to be lowered and thecrystalline particle becomes large. Therefore, it is appropriate thatAl₂O₃/MgO ratio is equal to or higher than 0.2, preferably equal to orhigher than 0.25.

TiO₂ is a nucleation agent for deposition of crystalline phase ofenstatite having a composition of MgO. SiO₂ and enstatite solid solutionhaving a composition of (Mg.Al)SiO₃. Moreover, if the content of SiO₂ isrelatively small, TiO₂ also has an effect of suppressing glassdevitrification. However, if the content of TiO₂ is less than 5%, theeffect as the nucleation agent for the main crystals cannot be obtainedsufficiently, thereby, crystallization causes at the surface of theglass and it tends to be difficult to produce homogenous crystallizedglass. On the other hand, if the content of TiO₂ exceeds 15%, phaseseparating and devitrification of the glass occur due to excessively lowglass viscosity at a high temperature, so that glass productivity tendsto extremely deteriorate. Therefore, it is appropriate that the contentof TiO₂ ranges from 5 to 15%, preferably from 5.5 to 14%. Morepreferably, it ranges from 8 to 14%.

It is to be noted that, as described below, it may be preferable thatthe content of TiO₂ ranges from 8.5 to 15% because a crystallized glasshaving the high Young's modulus equal to or higher than 160 GPa can beobtained by combination with other components if the Young's modulus isemphasized further than the surface roughness.

The crystallized glass of the present invention may contain Y₂O₃.However, as seen in the following examples, the Young's modulus of thecrystallized glass can be increased to about 10 GPa as well as thetemperature of liquid phase can be decreased to about from 50 to 100° C.by introducing, for example, 2% of Y₂O₃. That is, characteristics andproductivity of glass can be improved remarkably by introducing a littleamount of Y₂O₃. If the content of Y₂O₃ is equal to or higher than 0.3%,aforementioned effect of Y₂O₃ can be obtained. The content of Y₂O₃ispreferably equal to or higher than 0.5%. However, Y₂O₃ has an effect ofsuppressing the growth of the main crystal contained in theaforementioned glass. Therefore, if the content of Y₂O₃ is excessivelyhigh, the surface is easily crystallized in the heat-treatment carriedout for the purpose of the glass crystallization, so that it tends notto produce a desired crystallized glass. From such viewpoints, it isappropriate that the content of Y₂O₃ is equal to or less than 10%. Inparticular, the content of Y₂O₃ is preferably equal to or less than 8%,more preferably equal to or less than 3%.

Moreover, the crystallized glass of the present invention can containequal to or less than 10% of ZrO₂. ZrO₂ can play an important role toimprovement of glass stability, especially in a glass containing a largeamount of MgO. In addition, it also works as a nucleation agent and itcontributes, as a helper of TiO₂, to make the crystalline particles finewith promoting phase separation of glass during pretreatment. However,if the content of ZrO₂ exceeds 10%, it is afraid that meltingcharacteristics at a high temperature and homogeneity of glassdeteriorate, so that the amount to be introduced appropriately rangesfrom 1 to 10%. Moreover, the content of ZrO₂ preferably ranges from 0 to6%, more preferably from 1 to 5% in view of melting characteristics ofglass at a high temperature and homogeneity of the crystallineparticles.

In the crystallized glass of the present invention, the sum of SiO₂,Al₂O₃, MgO and TiO₂ is equal to or higher than 92% from the viewpoint ofcharacteristics such as the high Young's modulus and keeping homogenouscrystalline characteristics. The sum of SiO₂, Al₂O₃, MgO and TiO₂ ispreferably equal to or higher than 93%, more preferably equal to orhigher than 95%.

If within above range, as components other than said components, it maycontain such components as alkali metal oxides R₂O (for example, Li₂O,Na₂O, K₂O or the like) and/or alkali-earth metal oxides RO (for example,CaO, SrO, BaO or the like) in the range of not deteriorating desirablecharacteristics of the crystallized glass. Alkali metal oxides and/oralkali-earth metal oxides can be produced from nitrates as glass rawmaterials. If Sb₂O₃ is used as a degassing agent in glass producing, Ptis easily included into glass from a crucible made of Pt for glassmelting. By using nitrates as glass raw materials, inclusion of Pt intoglass can be suppressed. It is preferable that the content of alkalimetal oxides and alkali-earth metal oxides is respectively equal to orhigher than 0.1% from the view point of obtaining said effect. However,if containing alkali metal oxides, its content is appropriately equal toor less than 5% because alkali metal oxides tend to lower the Young'smodulus. On the other hand, alkali metal oxides have such effects aslowering a melting temperature of glass and melting the Pt contaminationfrom a melting crucible made of Pt by ionizing it. In this case, it isalso preferably to add them equal to or higher than 0.1% in order toenjoy these effects. Especially, K₂O is preferable because it has aneffect to suppress the reduction of the Young's modulus, as well aseffects of lowering a melting temperature of glass and melting the Ptcontamination from a melting crucible made of Pt by ionizing it. In thecase of containing K₂O, its content is appropriately equal to or lessthan 5%, preferably from 0.1 to 2%, more preferably from 0.1 to 1%

In addition, in the case of containing alkali-earth metal oxides, thecontent of the alkali-earth metal oxides is appropriately equal to orless than 5% because of their tendency to increase crystallineparticles. In the case of containing alkali metal oxides, especially,the content of K₂O preferably ranges from 0.1 to 5%, preferably from 0.1to 2% , more preferably from 0.1 to 1% . In the case of containingalkali-earth metal oxides, especially, the content of SrO ranges from0.1 to 5%, preferably from 0.1 to 2%. Especially, from the viewpoint ofglass stabilization, SrO is preferable and its content ranges from 0.1to 5%, preferably from 0.1 to 2%.

In addition, As₂O₃ and/or Sb₂O₃ can be contained as a degassing agentfor attempting glass homogenization. According to viscosity at a hightemperature changing by the composition of glass, more homogenous glasscan be obtained by adding an adequate amount of As₂O₃, Sb₂O₃ orAs₂O₃+Sb₂O₃ to glass. However, if excessively large amount of thedegassing agent is added, the Young's modulus tends to be lowered byincreasing the specific gravity of glass. Moreover, it may react with aplatinum crucible for melting, and thereby damaging the crucible.Therefore, it is appropriate that the amount of the degassing agentadded is equal to or less than 2%, preferably equal to or less than1.5%.

Impurities in raw materials, such as Cl, F, SO₃ and the like whichbecome a glass cleaning agent, can be contained other thanaforementioned basic components if the content is respectively equal toor less than 1% where deterioration of the characteristics of thecrystallized glass of the present invention is avoidable.

In addition, it is preferable that the crystallized glass of thepresents invention substantially does not contain ZnO and NiO. BecauseZnO facilitates production of spinel, hard crystals. In addition, notcontaining NiO is desirable from the viewpoint that NiO facilitatesproduction of spinel, as well as from the viewpoint that NiO is such acomponent as affecting the environment.

One of the preferred embodiments of the substrate for informationrecording medium of the present invention is a substrate composed ofcrystallized glass in which the main crystals are enstatite and/or itssolid solution, and comprising 35-65 mol % of SiO₂, 5-25 mol % of Al₂O₃,10-40 mol % of MgO, 5-15 mol % of TiO₂, 0-10 mol % of Y₂O₃, 0-6 mol % ofZrO₂, 0-5 mol % of R₂O where R represents at least one selected from thegroup consisting of Li, Na and K, 0-5 mol % of RO where R represents atleast one selected from the group consisting of Ca, Sr and Ba, 0-2 mol %of As₂O₃+Sb₂O₃ and equal to or higher than 92 mol % ofSiO₂+Al₂O₃+MgO+TiO₂.

In the present invention, the main crystals are essential crystals forobtaining effects of the present invention and the crystals contained inthe largest amount among crystals in glass. Preferably, the maincrystals mean the crystals contained equal to or higher than 50% byvolume among crystals in glass. It is to be noted that many crystallizedglasses of the present invention contain equal to or higher than 70% byvolume of enstatite and/or its solid solution, in some cases, equal toor higher than 80% by volume, in few cases, equal to or higher than 90%by volume. The main crystals in the crystallized glass of the presentinvention are, for example, enstatite (including enstatite solidsolution) having a composition of MgO SiO₂ and (Mg.Al) SiO₃. Inaddition, if the crystallized glass of the present invention containsboth of enstatite and its solid solution, a group of enstatite and itssolid solution is referred to as main crystals. It is to be noted thatenstatite includes clino-enstatite, proto-enstatite and enstatite.Moreover titanates can be included other than aforementioned crystals.Examples of crystalline phase include a phase in which the sum ofenstatite and/or its solid solution ranges from 50 to 100% by volume ofand titanates ranges from 50 to 0% by volume. In addition, a phase canbe exemplified in which the sum of enstatite and/or its solid solutionranges from 70 to 90% by volume of and titanates ranges from 30 to 10%by volume. In this case, the total amount of enstatite and/or its solidsolution and titanates is preferably equal to or higher than 90% byvolume, more preferably equal to or higher than 95% by volume, the mostpreferably equal to or higher than 99% by volume in crystals in glass.

As crystals other than enstatite and/or its solid solution as well astitanates, mullite, forsterite, cordierite, quartz, solid solution ofquartz and the like can be exemplified. However, spinel is not included.Enstatite has such characteristics as the crystallzed glass containingenstatite as main crystals can be polished very easily due to lowhardness (where the Mohs' scale of hardness is 5.5), and it can obtaindesirable surface roughness in relatively short time. Moreover, it canbe considered that enstatite can provide a glass with high Young'smodulus even if the particle size is small because glass componentseasily penetrate into its voids due to the crystal structure of chain orlayer. On the other hand, it is considered that because spinel is harder(where the Mohs' scale of hardness is 8) than enstatite, it makes glasshard to be polished. It is preferable that the crystallized glass of thepresent invention does not contain quartz solid solution.

In addition, the ratio of crystals in glass (degree of crystallinity)ranges from about 20 to 70% by volume. The degree of crystallinity ispreferably equal to or higher than 50% by volume in order to obtain sucha substrate as having high Young's modulus. However, in order to makeprocesses after crystallization easy, the degree of crystallinity mayrange from 20 to 50% by volume, moreover, from 20 to 30% by volume.Alternatively, when it is desirable to obtain high Young's modulusrather than facilitation of processes after crystallization, the degreeof crystallinity may range from 50 to 70% by volume.

Moreover, the average value of size of crystals contained in thecrystallized glass of the present invention (particle diameter) ispreferably equal to or less than 100 nm, more preferably equal to orless than 50 nm, further preferably equal to or less than 30 nm. If theaverage value of crystalline size exceeds 100 nm, not only lowering themechanical strength of glass but also the loss of crystals duringpolishing process causes, thereby it is afraid that the surfaceroughness of glass deteriorates. Such a control of crystalline size canbe done mainly by changing kinds of contained crystalline phase and heattreatment conditions mentioned below. In the present invention, it ispossible to obtain such micro crystalline size under the heat treatmentconditions under which the main crystals of enstatite and/or its solidsolution which are essential components in the present invention can beobtained.

In addition, the crystallized glass constituting the substrate of thepresent invention may have the thermal expansion coefficient rangingfrom 65×10⁻⁷ to 85×10 ⁷/° C., further from 73×10⁻⁷ to 83×10⁻⁷/° C. Thethermal expansion coefficient can be set within the above range in viewof characteristics required as a substrate for information recordingmedium.

[Production Method for Crystallized Glass and Substrate]

The substrate composed of the crystallized glass of the presentinvention can be produced by publicly known production methods of glasssubstrate. For example, glass materials of a given composition can bemelted by the high temperature melting method, i.e., melted in air orinert gas atmosphere, homogenized by bubbling, addition of degassingagent, stirring or the like and molded into plate glass by well-knownpress method, down draw method or the like. Then, glass molding productsof a desired size and shape can be obtained from the plate glass byprocessing such as cutting and polishing. The crystallized glass of thepresent invention can be melted, for example, at a temperature of from1400 to 1650° C., and includes such glasses as possible to be melted ata temperature of from 1500 to 1650° C., further from 1550 to 1600° C..As mentioned above, for example, K₂O is preferably introduced as acomponent for lowering a melting temperature.

The glass molding product thus obtained is subjected to heat treatmentmethod for crystallization. The heat treatment method is not especiallylimited and it can be selected properly according to the content ofcrystallization promoting agents, glass transition temperature,crystallization peak temperature or the like. However, many crystallinenuclei are generated by heat treatment at relatively low temperature(for example, (glass transition temperature (Tg)−30° C.) to (Tg+60° C.),especially, Tg to (Tg+60° C.) in the initial stage. The temperaturespecifically ranges from 700 to 850° C. After that, it is preferablethat the crystals are grown by elevating the temperature to from 850 to1150° C. from the viewpoint of obtaining micro crystals. At this time,after the glass temperature becomes ranging from 500 to 850° C., theelevating rate of temperature more preferably ranges from 0.1 to 10°C./min from the viewpoints of deposition of micro crystal particles andpreventing deformation of external form of plate glass. However, theelevating rate of temperature is not especially limited until the glasstemperature become ranging from 500 to 850° C., and it can be rangedfrom 5 to 50° C./min. In addition, in the present invention, theadmissible temperature range of heat treatment for producing crystallinenuclei and heat treatment for growth of crystals in which such acrystallized glass as having the same Young's modulus and the samecrystalline size or the same crystallization homogeneity can be producesis a temperature width equal to or higher than 30° C., so thatproduction process for crystallization can be controlled easily.

Moreover, in the present invention, such a heat treatment conditions areadjusted to conditions under which enstatite having a composition ofMgO.SiO₂ and enstatite solid solution having a composition of(Mg.Al)SiO₃ are deposited as main crystals. It is to be noted that othercrystals such as forsterite, cordierite, titanates and mullite may bedeposited other than these main crystals, but the conditions under whichenstatite and its solid solution are deposited is adopted. As for suchconditions, heat treatment for crystallization is preferably carried outat a temperature ranging from 850 to 1150° C. The heating is preferablycarried out at a temperature ranging from 875 to 1050° C. If the heatingtemperature is lower than 850° C., enstatite and its solid solution arehardly deposited. In addition, if exceeding 1150° C., crystals otherthan enstatite and its solid solution become easy to deposit. Inaddition, by setting the temperature from 875 to 1000° C., the particlesize of enstatite and/or its solid solution can be made relativelysmall, for example, equal to or less than 100 nm, preferably equal to orless than 50 nm. The heat treatment time for crystallization is selectedproperly according to desirable degree of crystallinity and particlediameter of crystals because it affects the degree of crystallinity andthe particle diameter of crystals depending on the heat treatmenttemperature. In the heat treatment at a temperature ranging from 850 to1150° C., it preferably ranges from 1 to 4 hours.

In addition, the temperature of nuclei producing process before heattreatment for crystallization appropriately ranges from 30° C. lower to60° C. higher than glass transition temperature (Tg), preferably from 0°C. to 60° C. higher than Tg, more preferably from 10 to 50° C. higherthan Tg from the viewpoint of deposition of crystals having smallcrystalline particle diameter.

The molding product of the crystallized glass after subjected to heattreatment may be polished, if necessary, and polishing method is notespecially limited. For example, it can be polished by publicly knownmethods using synthetic sharpening particle, such as synthetic diamond,silicon carbide, aluminum oxide and boron carbide, and naturalsharpening particle, such as natural diamond and cerium oxide. Themolded product which has been polished but not be subjected tocrystallization can also be subjected to the above-mentionedcrystallization.

The substrate for information recording medium composed of thecrystallized glass of the present invention preferably has the surfacesmoothness as having the average roughness Ra (JIS B0601) equal to orless than 1 nm measured with AFM (Atomic Force Microscopy). Especially,in the case of using the crystallized glass of the present invention asa magnetic disk substrate, the average roughness Ra (JIS B0601) of thesurface significantly influences recording density of the magnetic disk.If the surface roughness exceeds 1 nm, high recording density is hardlyobtained. The surface roughness of the substrate composed of thecrystallized glass of the present invention is more preferably equal toor less than 0.7 nm, further preferably equal to or less than 0.5 nm inview of obtaining high recording density.

The substrate composed of the crystallized glass of the presentinvention in which enstatite and its solid solution are contained asmain crystals is useful as a magnetic disk substrate because it has highstrength, high hardness and high Young's modulus as well as beingexcellent in chemical durability and heat resistance. Moreover, thecrystallized glass of the present invention is alkali free orlow-alkali, so that if used as a magnetic disk substrate, corrosion of amagnetic film with the substrate is greatly decreased, thereby themagnetic film can be kept in the best condition.

The crystallized glass of the present invention is SiO₂—Al₂O₃—MgO basedglass in which the main crystals are enstatite and/or its solidsolution, as mentioned above. Other than this, the present inventionincludes the crystallized glass substrates for information recordingmedium which are glasses other than SiO₂—Al₂O₃—MgO based glass and inwhich the main crystals are enstatite and/or its solid solution and inwhich s a polished plane having the surface roughness Ra (JIS B0601)equal to or less than 1 nm is provided. It is to be noted that the maincrystals herein are such crystals as contained in an amount equal to orhigher than 50% by volume among crystals in the glass. In manycrystallized glasses, enstatite and/or its solid solution are contained,in an amount equal to or higher than 70% by volume, in some cases, equalto or higher than 80% by volume, in few cases, equal to or higher than90% by volume of whole the crystals. In addition, in this crystallizedglass (crystallinity), the ratio of crystals in glass ranges about from20 to 70%.

In the crystallized glass substrate in this embodiment, the averagevalue of the size (particle diameter) of the crystals contained in thecrystallized glass is preferably equal to or less than 0.5 μm, morepreferably equal to or less than 0.3 μm, further preferably equal to orless than 0.1 μm. The average value of the size of the crystalscontained the crystallized glass is the most preferably equal to or lessthan 50 nm. If the average value of the crystalline size exceeds 0.5 μm,it is afraid that not only lowering the mechanical strength of the glassbut also the loss of crystals during polishing process causes, therebythe glass surface roughness deteriorates. Such a control of the size ofcrystal particles is carried out mainly by selecting kinds of thecontained crystalline phase and heat treatment condition mentionedbelow. However, in the present invention, it is possible to obtain themicro particle size as mentioned above in the heat treatment conditionsunder which the main crystals of enstatite and/or its solid solutionwhich are essential in the present invention are obtained.

Small crystal size results in an increase of light transparency at 600nm through the substrate with 1 mm thickness. The substrate of thepresent invention exhibits the transparency equal to or more than 10%,or equal to or more than 50%, in some cases, from 60% to 90%.

The crystallized glass substrate in this embodiment preferably has aYoung's modulus equal to or higher than 140 GPa as the substrate use forrotation at a high speed. In addition, it is preferable that thecrystallized glass substrate of the present invention substantially doesnot contain quartz solid solution as main crystals. Moreover, it ispreferable that the crystallized glass substrate of the presentinvention substantially does not contain spinel as a crystalline phase.Because spinel is hard crystals compared with enstatite (having theMohs' scale of hardness of 8), it is difficult to be polished and thepolished plane having a surface roughness Ra (JIS B0601) equal to orless than 1 nm is hardly obtained.

Demands for high Young's modulus can be explained on the basis of thefollowing fact. That is, with recent smaller size, higher capacity andhigher speed of HDDs, it is expected that the thickness of 3.5-inchdisks currently used of 0.8 mm will be made smaller to 0.635 mm, and0.635 mm of current 2.5 inch disks to 0.4 3 mm, or even to 0.38 mm.Revolution speed of substrates is also expected to be made faster fromthe current maximum speed of 7,200 rpm to 10,000 rpm, or even to 14,000rpm. As substrates for such magnetic recording media become thinner,they become more likely to suffer deflexion, undulation and warp, and itis expected that, as the revolution speed becomes higher, stress loadedon the substrates (force exerted by wind pressure caused by rotation ofdisks) will become larger. Based on the theory of dynamics, thedeflexion W of a disk receiving load of P per unit area is representedby the following formula: $W\quad\infty\quad\frac{{Pa}^{4}}{h^{3}E}$wherein a represents an outer diameter of disk, h represents a thicknessof substrate and E represents Young's modulus of disk material.

In static state, force loaded on the disk is the gravitation alone is,and the deflexion W is represented by the following formula:${W \propto \frac{h\quad{da}^{4}}{h^{3}E}} = {\frac{{da}^{4}}{h^{2}E} = \frac{a^{4}}{h^{2}G}}$wherein d represents a specific gravity of disk material and G is aspecific elastic modulus of disk material (=Young's modulus/specificgravity).

On the other hand, supposing that the gravitational force is balanced bycentrifugal force and can be ignored in rotating state of disk, forceloaded on the disk may be considered only wind pressure caused by therotation of the disk. The wind pressure is represented as a function ofdisk revolution speed and said to be proportional to the second power ofthe speed. Accordingly, the deflexion W when the disk is rotating isrepresented by the following formula:$w \propto \frac{({rpm})^{2}a^{4}}{h^{3}E}$

From these results, in order to suppress the deflexion W of substrate tobe rotated at a high speed, a material of high Young's modulus E isrequired. According to the present inventors' calculation, when thethickness of 2.5-inch substrate is made smaller from 0.635 mm to 0.43mm, and the thickness of 3.5-inch substrate from 0.8 mm to 0.635 mm, itis required that the specific elastic modulus of substrate materials isat least equal to or higher than 37 MNm/kg. In addition, when thecurrent revolution speed of 3.5-inch high-end substrates of 7200 rpm ismade faster to prospective 10000 rpm, an aluminum substrate havingYoung's modulus of around 70 GPa cannot meet such a high speed, and newsubstrate materials having the Young's modulus at least equal to orhigher than GPa are required. As the specific elastic modulus or Young'smodulus of substrate material becomes higher, not only stiffness ofsubstrates becomes higher, but also impact resistance and strength ofsubstrates become higher. Therefore, a glass material having highspecific elastic modulus and high Young's modulus is strongly desired inthe field of HDD production.

In the present invention, such a substrate has high Young's modulusequal to or higher than 160 GPa, as comprising 35 to 43 mol % of SiO₂, 9to 20 mol % of Al₂O₃, 30 to 39 mol % of MgO, 1 to 3 mol % of Y₂O₃, 8.5to 15 mol % of TiO₂ and 1 to 5 mol % of ZrO₂. In this case, the molarratio of SiO₂/MgO is appropriately equal to or less than 1.35.

The magnetic disk substrate composed of the crystallized glass of thepresent invention can satisfy all characteristics required as a magneticdisk substrate, such as surface smoothness, flatness, strength,hardness, chemical durability and heat resistance. In addition, theYoung's modulus is about twice larger than the conventional crystallizedglass (Li₂O—SiO₂ based crystallized glass), so that it is possible tosuppress the deflexion caused by high revolution speed of the disk low,thereby it is suitable as a substrate material to achieve high TPI harddisks.

Because the crystallized glass of the present invention is excellent inheat resistance, surface smoothness, chemical durability, opticalcharacteristics and mechanical strength, it can be used suitable assubstrates for information recording medium such as magnetic disks,glass substrates for optical magnetic disks and electron optical glasssubstrates such as optical disks.

[Explanation of Magnetic Disk]

The information recording medium of the present invention ischaracterized by having the substrate of the present invention and arecording layer formed on said substrate. A magnetic disk (hard disk)comprising a substrate composed of the crystallized glass of the presentinvention described above and at least a magnetic layer formed on a mainsurface of the substrate will be explained hereinafter. As layers otherthan the magnetic layer, underlying layer, protective layer, lubricatinglayer, unevenness control layer and the like are optionally formeddepending on functions of the disk. These layers can be formed byvarious thin film-forming techniques.

Material for the magnetic layer is not particularly limited. Forexample, in addition to Co magnetic layers, ferrite magnetic layers,iron-rare earth metal magnetic layers and the like can be mentioned. Themagnetic layer may be either for horizontal magnetic recording orvertical magnetic recording. Specific examples of the magnetic layerinclude, for example, those containing Co as a main component such asCoPt, CoCr, CoNi, CoNiCr, CoCrTa and CoPtCr, and CoNiCrPt, CoNiCrTa,CoCrPtTa, CoCrPtSiO and the like. The magnetic layer may be consisted ofmultiple layers comprising a non-magnetic layer for noise reductionseparating magnetic layers. The underlying layer of the magnetic layermay be selected depending on the nature of the magnetic layer. Forexample, the underlying layer may be those comprising one or more ofnon-magnetic metals such as Cr, Mo, Ta, Ti, W, V, B and Al, or oxides,nitride, carbides and the like of those metals. For a magnetic layercomprising Co as the main component, an underlying layer of pure Cr orCr alloy is preferred for improving magnetic characteristics. Theunderlying layer is not limited to a monolayer, and may be composed ofmultiple layers consisting of multiple identical or different layers.For example, the underlying layer may be a multi-layer underlying layersuch as Al/Cr/CrMo and Al/Cr/Cr.

The unevenness control layer for preventing absorption of magnetic diskto magnetic head may be provided between the substrate and the magneticlayer or on the magnetic layer. Because surface roughness of the disk isproperly controlled by the unevenness control layer, the magnetic diskis prevented from being absorbed to the magnetic disk and hence a highlyreliable magnetic disk can be provided. Various materials and productionmethods for the unevenness control layer have been known and they arenot particularly limited. For example, the material of the unevennesscontrol layer may be one or more metals selected from Al, Ag, Ti, Nb,Ta, Bi, Si, Zr, Cr, Cu, Au, Sn, Pd, Sb, Ge, Mg and the like, alloysthereof, oxides, nitrides, carbides thereof and the like. For the easeof production, those produced from metals containing Al as a maincomponent such as pure Al, Al alloys, Al oxides and Al nitrides arepreferred.

In addition, for good head stiction, surface roughness of the unevennessforming layer is preferably R_(max) of 50-300 Å, more preferably R_(max)of 100-200 Å. When the R_(max) is less than 50 Å, the disk surface isnearly flat, and hence the magnetic head and the disk are absorbed toeach other. This may disadvantageously cause damage of the magnetic headand the magnetic disk, and head crash. On the other hand, when theR_(max) exceeds 300 Å, glide height becomes larger and recording densityis disadvantageously lowered.

It is to be noted that unevenness may be provided on the surface of theglass substrate by a texturing treatment such as etching treatment andirradiation of laser lights instead of providing the unevenness controllayer.

The protective layer may be, for example, Cr layer, Cr alloy layer,carbon layer, zirconia layer, silica layer or the like. These protectivelayers can be successively formed by an inline sputtering apparatustogether with the underlying layer, the magnetic layer and the like.These protective layers may have either monolayer structure ormultilayer structure comprising identical or different layers.

Another protective layer may be provided on or instead of the protectivelayer explained above. For example, a silicon oxide (SiO₂) layer may beformed on the protective layer mentioned above by applyingtetraalkoxysilane diluted in an alcoholic solvent, in which colloidalsilica is further dispersed, and sintering the applied layer. This layerfunctions as a protective layer and as an unevenness control layer.

While various kinds of layers have been proposed as the lubricatinglayer, it is generally formed by applying a liquid lubricating agent,perfluoropolyether, diluted in a solvent such as freon by dipping, spincoating, spraying or the like and subjecting the coated layer to aheat-treatment as required.

EXAMPLES

The details of the present invention will be explained in the followingexamples, however, the present invention is not limited to theseexamples.

In Tables 1 to 12, the glass compositions of Examples 1 to 42 are listedwith respect to mol % (or % by weight). The compositions shown in Tables1 to 12 are those of the starting materials. However, as the results ofanalysis on the crystallized glass of Examples 1 to 15, thecompositional difference between the starting material and thecrystallized glass is within ±0.1%. Thus, the compositions of thestarting materials shown in Tables 1 to 12 are substantially identicalwith these of the crystallized glasses. As the starting materials ofthese glasses, SiO₂, Al₂O₃, Al(OH)₃, MgO, Y₂O₃, TiO₂, ZrO₂, KNO₃,Sr(NO₃)₂, Sb₂O₃ and the like were weighed into 250-300 g portionsaccording to the given compositions shown in Tables 1 to 12 and mixedsufficiently to provide formulated batches. Provided that, not shown inthe tables, all of the glasses contain 0.03 mol % of Sb₂O₃. Each of themwas charged in a platinum crucible and melted in air for 4 to 5 hours at1550° C. with stirring. After melting, the glass melting liquid was castinto a carbon mold having a size of 180×15×25 mm, left to cool to theglass transition temperature, immediately transferred into an annealingfurnace, annealed in the glass transition temperature range for about 1hour and left to cool to room temperature in the furnace. The resultingglasses did not contain deposited crystals which can be observed by amicroscope. After glass pieces having a size of 180×15×25 mm werepolished into pieces having a size of 100×10×10 mm or 10×10×20 mm, theywere put in a heat treatment furnace, elevating a temperature at anelevating speed of temperature of from 1 to 5° C./min. up to thetemperature of heat treatment for crystal nucleus production describedin Tables 1 to 12, and immediately after finishing the heat treatmentfor crystal nucleus production, elevating a temperature at an elevatingspeed of temperature of from 2 to 10° C./min. from the temperature ofheat treatment for crystal nucleus production to the temperature of heattreatment for crystallization described in Tables 1 to 12. After keepingthe temperature for 1 to 5 hours, the crystallized glass was produced bycooling to the room temperature in the furnace. The crystallized glassthus obtained was further polished as to have the length of 95 mm, andthen used as a sample for measuring the Young's modulus and the specificgravity. The data obtained by the measurements are listed together withthe glass compositions in Tables 1 to 12.

For comparison, the ion-exchanged glass substrate disclosed in JapanesePatent Unexamined Publication No. Hei.1-239036 (JP-A-239036/89) and theglass substrate disclosed in U.S. Pat. No. 2,516,553 were utilized asComparative examples 1 and 2 and their compositions and characteristicsare shown in Table 13. TABLE 1 Composition(mol %) 1 2 3 SiO₂ 48.00 47.0047.00 Al₂O₃ 11.00 10.50 10.50 MgO 30.00 30.00 28.50 Y₂O₃ 1.00 0.50 ZrO₂2.00 2.00 TiO₂ 10.00 10.00 10.00 Al₂O₃/MgO 0.37 0.35 0.37 SiO₂/MgO 1.601.56 1.65 S + A + M + T 99 97.5 96 Transition temperature Tg (° C.) 732735 729 Heat treatment temperature of Tg + 28 Tg + 35 Tg + 31 producingcrystal nucleus(° C.) Heat treatment time of 4 4 4 producing crystalnucleus(h) Elevating speed 300 300 300 of temperature(° C./h)* Heattreatment temperature 1000 1000 1000 of crystallization(° C.) Heattreatment time 4 4 4 of crystallization(h) Elevating speed 240 240 240of temperature(° C./h)** Kind of fracture surface Glass Glass GlassTransparency at a 75% 73% 78% wavelength of 600 nm Specificgravity(g/cm³) 3.086 3.14 3.11 Young's modulus(GPa) 148.5 150.5 147Poisson ratio 0.231 0.23 0.229 Kind of main crystals Enstatite EnstatiteEnstatite Kind of other crystals Titanate Titanate Titanate Specificmodulus 48.1 47.9 47.3 of elasticity(MNm/kg) Thermal expansion 72 72 75coefficient(10⁻⁷/° C.) Average particle diameter(nm) 30˜40 30˜40 40˜50Ra(nm) 0.3 0.3 0.3Enstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 2 Composition(mol %) 4 5 6 SiO₂ 46.00 47.00 41.00 Al₂O₃ 10.5012.50 12.50 MgO 31.00 28.50 34.50 Y₂O₃ 0.50 2.00 2.00 ZrO₂ 2.00 TiO₂10.00 10.00 10.00 Al₂O₃/MgO 0.34 0.44 0.36 SiO₂/MgO 1.48 1.65 1.18 S +A + M + T 97.5 98 98 Transition temperature Tg (° C.) 732 729 738 Heattreatment temperature of Tg + 38 Tg + 31 Tg + 22 producing crystalnucleus(° C.) Heat treatment time of 4 4 4 producing crystal nucleus(h)Elevating speed 300 300 300 of temperature(° C./h)* Heat treatmenttemperature 1000 1000 1000 of crystallization(° C.) Heat treatment time4 4 4 of crystallization(h) Elevating speed 240 240 240 of temperature(°C./h)** Kind of fracture surface Glass Glass Glass Transparency at a 79%76% 75% wavelength of 600 nm Specific gravity(g/cm³) 3.158 3.038 3.309Young's modulus(GPa) 153.2 146.7 179.1 Poisson ratio 0.23 0.225 0.245Kind of main crystals Enstatite Enstatite Enstatite Kind of othercrystals Titanate Titanate Titanate Specific modulus 48.5 48.3 54.1 ofelasticity(MNm/kg) Thermal expansion 74 70 83 coefficient(10⁻⁷/° C.)Average particle diameter (nm) 20˜30 50˜70 100˜150 Ra(nm) 0.25 0.4 0.5Enstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 3 Composition(mol %) 7 8 9 10 SiO₂ 43.00 45.00 47.00 49.00 Al₂O₃12.50 12.50 12.50 10.50 MgO 32.50 30.50 28.50 29.50 Y₂O₃ 2.00 2.00 2.001.00 ZrO₂ TiO₂ 10.00 10.00 10.00 10.00 Al₂O₃/MgO 0.38 0.41 0.44 0.35SiO₂/MgO 1.32 1.47 1.65 1.66 S + A + M + T 98 98 98 99 Transitiontemperature Tg (° C.) 740 739 740 732 Heat treatment temperature of Tg +30 Tg + 31 Tg + 20 Tg + 28 producing crystal nucleus(° C.) Heattreatment time of producing crystal nucleus(h) 4 4 4 4 Elevating speedof temperature(° C./h)* 300 300 300 300 Heat treatment temperature ofcrystallization(° C.) 1000 1000 1000 1000 Heat treatment time ofcrystallization(h) 4 4 4 4 Elevating speed of temperature(° C./h)** 240240 240 240 Kind of fracture surface Glass Glass Glass GlassTransparency at a wavelength of 600 nm 73% 78% 79% 76% Specificgravity(g/cm³) 3.254 3.207 3.168 3.068 Young's modulus(GPa) 170 163.4157.3 149.1 Poisson ratio 0.245 0.241 0.237 0.228 Kind of main crystalsEnstatite Enstatite Enstatite Enstatite Kind of other crystals TitanateTitanate Titanate Titanate Specific modulus of elasticity(MNm/kg) 52.251.0 49.7 48.6 Thermal expansion coefficient(10⁻⁷/° C.) 81 76 72 77Average particle diameter(nm) 80˜120 50˜70 50˜70 40˜50Enstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 4 Composition(mol %) 11 12 13 SiO₂ 46.00 46.00 46.00 Al₂O₃ 10.5010.50 10.50 MgO 30.50 30.00 30.00 K₂O 0.50 SrO 1.00 1.50 Y₂O₃ 0.50 0.50ZrO₂ 2.00 2.00 2.00 TiO₂ 10.00 10.00 9.00 Al₂O₃/MgO 0.34 0.35 0.35SiO₂/MgO 1.51 1.53 1.53 S + A + M + T 97 96.5 95.5 Transitiontemperature Tg(° C.) 726 728 726 Heat treatment temperature of Tg + 30Tg + 30 Tg + 30 producing crystal nucleus(° C.) Heat treatment time of 44 4 producing crystal nucleus(h) Elevating speed 300 300 300 oftemperature(° C./h)* Heat treatment temperature 1000 1000 1000 ofcrystallization(° C.) Heat treatment time 4 4 4 of crystallization(h)Elevating speed 240 240 240 of temperature(° C./h)** Kind of fracturesurface Glass Glass Glass Transparency at a 82% 70% 67% wavelength of600 nm Specific gravity(g/cm³) 3.127 3.172 3.175 Young's modulus(GPa)149.2 151.8 152.1 Poisson ratio 0.232 0.234 0.234 Kind of main crystalsEnstatite Enstatite Enstatite Kind of other crystals Titanate TitanateTitanate Specific modulus 47.7 47.9 47.9 of elasticity(MNm/kg) Thermalexpansion 77.9 75.1 74.7 coefficient(10⁻⁷/° C.) Average particlediameter(nm) 20˜30 30˜50 30˜50 Ra(nm) 0.20 0.30 0.30Enstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 5 Composition(mol %) 14 15 16 SiO₂ 46.00 46.00 46.00 Al₂O₃ 10.5010.50 10.50 MgO 31.00 30.00 31.0 K₂O 0.50 0.50 SrO 1.00 0.50 Y₂O₃ 0.500.50 0.50 ZrO₂ 2.00 2.00 2.00 TiO₂ 9.00 10.00 9.50 Al₂O₃/MgO 0.34 0.340.34 SiO₂/MgO 1.48 1.53 1.48 S + A + M + T 96.5 96.5 97 Transitiontemperature Tg(° C.) 725 727 734 Heat treatment temperature of Tg + 30Tg + 30 Tg + 26 producing crystal nucleus(° C.) Heat treatment time of 44 4 producing crystal nucleus(h) Elevating speed 300 300 300 oftemperature(° C./h)* Heat treatment temperature 1000 1000 1000 ofcrystallization(° C.) Heat treatment time 4 4 4 of crystallization(h)Elevating speed 240 240 240 of temperature(° C./h)** Kind of fracturesurface Glass Glass Glass Transparency at a 80% 80% 78% wavelength of600 nm Specific gravity(g/cm³) 3.124 3.15 3.139 Young's modulus(GPa)147.1 148.2 149.5 Poisson ratio 0.231 0.232 0.229 Kind of main crystalsEnstatite Enstatite Enstatite Kind of other crystals Titanate TitanateTitanate Specific modulus 47.1 47.0 47.6 of elasticity(MNm/kg) Thermalexpansion 79.2 78.1 75 coefficient(10⁻⁷/° C.) Average particlediameter(nm) 20˜30 20˜30 20˜30 Ra(nm) 0.30 0.25 0.20Enstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 6 Composition (mol %) 17 18 19 20 mol % wt % mol % wt % mol % wt %mol % wt % SiO₂ 39.00 35.57 39.00 35.13 39.00 36.07 39.00 34.76 Al₂O₃12.50 19.35 12.50 19.11 11.00 17.27 14.00 19.41 MgO 33.50 20.49 32.5019.63 35.00 21.72 32.00 21.17 Y₂O₃ 2.00 6.86 2.00 6.77 2.00 6.95 2.006.88 TiO₂ 10.00 12.13 10.00 11.98 10.00 12.30 10.00 12.16 ZrO₂ 3.00 5.614.00 7.39 3.00 5.69 3.00 5.63 Tg 743 744 741 746 Al₂O₃/MgO 0.373 0.3850.314 0.438 SiO₂/MgO 1.16 1.2 1.114 1.22 S + A + M + T 95 94 95 95Transparency at 600 nm(%) 40 40 48 30 Average particle (nm) 120-200120-200 100-150 200-250 Ra(nm) 0.50 0.50 0.40 0.70 Expansion(10⁻⁷/° C.)78 77 75 74 Young's modulus(GPa) 175.00 168.90 177.00 172.90 Poissonratio ? 0.250 0.251 0.250 0.251 Specific gravity(g/cm³) 3.401 3.3893.425 3.349 Specific modulus of elasticity(MNm/kg) 51.5 49.8 51.7 51.6Tn 790° C./4 h 790° C./4 h 790° C./4 h 790° C./4 h Elevating speed oftemperature* 5° C./min 5° C./min 5° C./min 5° C./min Tc 1000° C./4 h1000° C./4 h 1000° C./4 h 1000° C./4 h Elevating speed of temperature**5° C./min 5° C./min 5° C./min 5° C./min State of crystallization GoodGood Good Good Kind of main crystals Enstatite Enstatite EnstatiteEnstatite Kind of other crystals Titanate Titanate Titanate TitanateEnstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 7 Composition (mol %) 21 22 23 24 mol % wt % mol % wt % mol % wt %mol % wt % SiO₂ 39.00 35.89 39.00 36.60 38.00 34.76 38.00 34.76 Al₂O₃12.50 19.52 12.50 19.91 12.50 19.41 12.50 19.41 MgO 35.00 21.61 34.5021.72 34.50 21.17 34.50 21.17 Y₂O₃ 2.00 6.92 1.00 3.53 2.00 6.88 2.006.88 TiO₂ 8.50 10.40 10.00 12.48 10.00 12.16 10.00 12.16 ZrO₂ 3.00 5.663.00 5.77 3.00 5.63 3.00 5.63 Tg 750 741 742 Al₂O₃/MgO 0.357 0.362 0.3620.362 SiO₂/MgO 1.114 1.13 1.13 1.101 S + A + M + T 94.5 96 95 95Transparency at 600 nm(%) 35 40 40 25 Average particle(nm) 150-220120-200 120-200 150-200 Ra(nm) 0.70 0.40 0.40 0.65 Expansion(10⁻⁷/° C.)73 75 74 78 Young's modulus(GPa) 175.70 182.50 175.40 188.50 Poissonratio ? 0.256 0.246 0.255 0.245 Specific gravity(g/cm³) 3.404 3.3813.416 3.435 Specific modulus of elasticity(MNm/kg) 51.6 54.0 51.3 54.9Tn 790° C./4 h 790° C./4 h 790° C./4 h 790° C./4 h Elevating speed oftemperature* 5° C./min 5° C./min 5° C./min 5° C./min Tc 1000° C./4 h1000° C./4 h 1000° C./4 h 1000° C./4 h Elevating speed of temperature**5° C./min 5° C./min 5° C./min 5° C./min State of crystallization GoodGood Good Good Kind of main crystals Enstatite Enstatite EnstatiteEnstatite Kind of other crystals Titanate Titanate Titanate TitanateEnstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 8 Composition(mol %) 25 26 27 28 SiO₂ 58.00 55.00 47.00 48.00Al₂O₃ 11.00 10.50 20.00 15.00 MgO 20.00 24.00 22.50 25.50 K₂O SrO Y₂O₃0.50 0.50 0.50 ZrO₂ 1.00 1.00 TiO₂ 10.00 10.00 10.00 10.00 Al₂O₃/MgOSiO₂/MgO 2.9 2.29 2.09 1.88 S + A + M + T 99 99.5 99.5 98.5 Tg(° C.) 740735 745 740 Heat treatment temperature Tg + 30 Tg + 25 Tg + 25 Tg + 30of producing crystal nucleus(° C.) Heat treatment of producing crystalnucleus(h) 2 2 2 2 Elevating speed of temperature(° C./h)* 300 300 300300 Heat treatment temperature of crystallization(° C.) 1000 1000 10001000 Heat treatment time of crystallization(h) 4 4 4 4 Elevating speedof temperature(° C./h)** 300 300 300 300 Fracture surface Glass GlassGlass Glass Transparency at a wavelength of 600 nm 72% 76% 35% 55%Specific gravity(g/cc) 3.12 3.14 3.05 3.08 Young's modulus(GPa) 145 149139 142 Poisson ratio 0.221 0.22 0.231 0.232 Specific modulus ofelasticity(MNm/kg) 46.5 47.5 45.6 46.1 Kind of main crystals EnstatiteEnstatite Enstatite Enstatite Kind of other crystals Titante TitanteTitante Titante Thermal expansion coefficient(10⁻⁷/° C.) 68 72 62 65Average particle diameter(nm) 20-30 20-30 50-70 30-50 Ra(nm) 0.3 0.3 0.50.4Enstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 9 Composition(mol %) 29 30 31 32 SiO₂ 38.00 46.00 46.00 39.00Al₂O₃ 8.00 10.50 10.50 11.00 MgO 36.50 28.00 27.00 35.00 K₂O 4.00 1.00SrO 3.50 Y₂O₃ 5.00 0.50 0.50 1.00 ZrO₂ 3.00 2.00 2.00 1.00 TiO₂ 9.509.00 9.50 13.00 Al₂O₃/MgO SiO₂/MgO 1.04 1.64 1.70 1.11 S + A + M + T 9293 93 98 Tg(° C.) 730 715 735 741 Heat treatment temperature Tg + 50Tg + 35 Tg + 25 Tg + 29 of producing crystal nucleus(° C.) Heattreatment of producing crystal nucleus(h) 2 2 2 2 Elevating speed oftemperature(° C./h)* 300 300 300 300 Heat treatment temperature ofcrystallization(° C.) 1000 1000 1000 1000 Heat treatment time ofcrystallization(h) 4 4 4 4 Elevating speed of temperature(° C./h)** 300300 300 300 Fracture surface Glass Glass Glass Glass Transparency at awavelength of 600 nm 30% 75% 72% 62% Specific gravity(g/cc) 3.41 3.0123.124 3.39 Young's modulus(GPa) 191 138 146 182 Poisson ratio 0.2410.223 0.221 0.242 Specific modulus of elasticity(MNm/kg) 56.0 45.8 46.753.7 Kind of main crystals Enstatite Enstatite Enstatite Enstatite Kindof other crystals Titante Titante Titante Titante Thermal expansioncoefficient(10⁻⁷/° C.) 82 72 74 81 Average particle diameter(nm) 50-8015-30 15-30 30-60 Ra(nm) 0.4 0.2 0 .2 0.4Enstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 10 Composition(mol %) 33 34 35 SiO₂ 46.00 46.00 46.00 Al₂O₃ 10.5010.50 10.50 MgO 30.50 30.50 30.50 K₂O 0.50 0.50 0.50 Y₂O₃ 0.50 0.50 0.50ZrO₂ 2.00 2.00 2.00 TiO₂ 10.00 10.00 10.00 Al₂O₃/MgO 0.34 0.34 0.34SiO₂/MgO 1.51 1.51 1.51 S + A + M + T 97 97 97 Transition temperatureTg(° C.) 728 728 728 Heat treatment temperature of 770 750 780 producingcrystal nucleus(° C.) Heat treatment time of 1 1 1 producing crystalnucleus(h) Elevating speed 1200 1200 1200 of temperature(° C./h)* Heattreatment temperature 980 980 980 of crystallization(° C.) Heattreatment time 4 4 4 of crystallization(h) Elevating speed 300 300 300of temperature(° C./h)** Kind of fracture surface Glass Glass GlassTransparency at a 72% 72% 72% wavelength of 600 nm Specificgravity(g/cm³) 3.115 3.113 3.113 Young's modulus(GPa) 146.2 146.0 146.1Poisson ratio 0.232 0.231 0.232 Kind of main crystals EnstatiteEnstatite Enstatite Kind of other crystals Titanate Titanate TitanateSpecific modulus 46.9 46.9 46.9 of elasticity(MNm/kg) Thermal expansion73 73 73 coefficient(10⁻⁷/° C.) Average particle diameter(nm) 20-3020-30 20-30 Ra(nm) 0.20 0.20 0.20Enstatite: Enstatite and its solid solutionS + A + M + T. = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 11 Composition(mol %) 36 37 38 SiO₂ 46.00 46.00 46.00 Al₂O₃ 10.5010.50 10.50 MgO 30.50 30.50 30.50 K₂O 0.50 0.50 0.50 Y₂O₃ 0.50 0.50 0.50ZrO₂ 2.00 2.00 2.00 TiO₂ 10.00 10.00 10.00 Al₂O₃/MgO 0.34 0.34 0.34SiO₂/MgO 1.51 1.51 1.51 S + A + M + T 97 97 97 Transition temperatureTg(° C.) 728 728 728 Heat treatment temperature of 800 770 770 producingcrystal nucleus(° C.) Heat treatment time of 1 0.5 0.5 producing crystalnucleus(h) Elevating speed 1200 1200 1200 of temperature(° C./h)* Heattreatment temperature 980 980 980 of crystallization(° C.) Heattreatment time 4 1 3 of crystallization(h) of raising of temperature 300300 300 (° C./h)** Kind of fracture surface Glass Glass GlassTransparency at a 72% 76% 72% wavelength of 600 nm Specificgravity(g/cm³) 3.111 3.104 3.110 Young's modulus(GPa) 146.0 145.5 145.4Poisson ratio 0.231 0.232 0.232 Kind of main crystals EnstatiteEnstatite Enstatite Kind of other crystals Titanate Titanate TitanateSpecific modulus 46.9 46.9 46.8 of elasticity(MNm/kg) Thermal expansion73 73 73 coefficient(10⁻⁷/° C.) Average particle diameter(nm) 20-3020-30 20-30 Ra(nm) 0.20 0.20 0.20Enstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 12 Composition(mol %) 39 40 41 42 SiO₂ 46.00 46.00 46.00 46.00Al₂O₃ 10.50 10.50 10.50 10.50 MgO 30.50 30.50 30.50 30.50 K₂O 0.50 0.500.50 0.50 Y₂O₃ 0.50 0.50 0.50 0.50 ZrO₂ 2.00 2.00 2.00 2.00 TiO₂ 10.0010.00 10.00 10.00 Al₂O₃/MgO 0.34 0.34 0.34 0.34 SiO₂/MgO 1.51 1.51 1.511.51 S + A + M + T 97 97 97 97 Transition temperature Tg(° C.) 728 728728 728 Heat treatment temperature of 770 770 700 700 producing crystalnucleus(° C.) Heat treatment time of producing crystal nucleus(h) 0.50.5 2 2 Elevating speed of temperature(° C./h)* 1200 1200 1200 1200 Heattreatment temperature of crystallization(° C.) 875 970 970 1025 Heattreatment time of crystallization(h) 2 2 2 2 Elevating speed oftemperature(° C./h)** 300 300 300 300 Kind of fracture surface GlassGlass Glass Glass Transparency at a wavelength of 600 nm 75% 75% 75% 75%Specific gravity(g/cm³) 3.057 3.101 3.111 3.111 Young's modulus(GPa)137.8 144.1 144.70 144.70 Poisson ratio 0.233 0.233 0.221 0.221 Kind ofmain crystals Enstatite Enstatite Enstatite Enstatite Kind of othercrystals Titanate Titanate Titanate Titanate Specific modulus ofelasticity(MNm/kg) 45.1 46.5 46.5 46.5 Thermal expansioncoefficient(10⁻⁷/° C.) 71 73 73 73 Average particle diameter(nm) 15-2020-30 20-30 20-30 Ra(nm) 0.20 0.30 0.30 0.30Enstatite: Enstatite and its solid solutionS + A + M + T = SiO₂ + Al₂O₃ + MgO + TiO₂*Elevating speed of temperature in the process of elevating temperatureup to heat treatment temperature of producing crystal nucleus**Elevating speed of temperature in the process of elevating temperaturefrom heat treatment temperature of producing crystal nucleus tocrystallization heat treatment temperature

TABLE 13 Comparative Example 1 2 Chemically Commercially tempered glassavailable TS-10 Japanese Patent crystallized Unexamined PublicationNo.glass Hei. 1-239036 U.S. Pat. No. Oxide (JP-A-239036/89) 2516553 SiO₂73.0 Al₂O₃ 0.6 CaO 7.0 Na₂O 9.0 K₂O 9.0 ZnO 2.0 As₂O₃ 0.2 Young'smodulus(GPa) 79 90-100 Surface roughness 12 10-35  Ra(nm)(1) Determination of Kinds of Crystals

The X-ray diffraction was measured using X-ray of Cu with respect to apowder forming from glass after crystallization (apparatus: X-raydiffraction apparatus MXP18A manufactured by Mac Science Co. Ltd., tubevoltage: 50 kV, tube current: 300 mA, scanning angle: 10 to 90). Thedeposited crystals were determined from peaks observed in the resultingX-ray diffraction.

(2) Measurement Method of Physical Characteristics Measurement ofSpecific Gravity (Density)

The crystallized glass sample itself was used as a sample for thespecific gravity measurement. An electron specific gravity meter(MD-200S manufactured by Mirage trade Co. Ltd.) utilizing the Archimedesmethod was used. The specific gravity was determined at room temperaturewith accuracy of ±0.001 g/cm³.

Measurement of Young's Modulus

Before measuring the Young's modulus, with a sample with parallelsurfaces having the end area of from 10 mm squares to 20 mm squares andthe length of from 50 to 100 mm, the specific gravity (density) wasmeasured and the sample length was measured with a slide caliper, thenthese were used as measurement conditions. UVM-2 manufactured byUltrasonic wave Industry Co. Ltd. was used. When the longitudinal wave(T11, T12) and the transverse wave (TS1, TS2) were measured, “water” inthe case of longitudinal wave and “Sonicoat SHN20 or SHN-B25” in thecase of transverse wave were coated between the deep probe and sampleend side. The same sample was subjected to the repeatedly measurement intwice or more times for the longitudinal wave and fifth or more timesfor the transverse wave, then calculating the average values. It is tobe noted that the Poisson ratio was obtained at the same time by thisoperation. The elastic modulus and Poisson's ratio were examined withaccuracies of ±1 GPa and ±0.001.

Thermal Mechanical Analysis

Test pieces were cut off from the crystallized glass aftercrystallization, subjected to polishing process as to form a columnshape of φ5 mm×20 mm, then they were used as samples for TMAmeasurement. TAS100 manufactured by Rigaku Co. Ltd. was used as ameasurement apparatus. The measurement conditions were set as theelevating speed of temperature was 4 K/min. and the maximum temperaturewas 350° C.

Atomic Force Microscopy

The crystallized glass samples were processed as to form pieces having asize of 30×2×1 mm, and subjected to optical precise polishing to givetwo plane surfaces having a size of 30×15 mm, then they were used assamples for AFM measurement. Nano Scope III manufactured by DigitalInstrument Co. Ltd. was used. As for the measurement conditions, themeasurement area was set as 2×2 μm or 5×5 μm, the number of samples wasset as 256×256 and scan rate was set as 1 Hz with Tapping mode AFM aswell as the data processing conditions were set as Planefit Auto order 3(X, Y) and Flatten Auto order 3. Integral grain, Proportion gain and Setpoint were adjusted at each measurement. It is to be noted that, aspretreatment of the measurement, the polished samples were washed withpure water, IPA or the like in a large washing machine in a clean room.

Transparency Measurement

The piece having a thickness of 1 mm subjected to optical precisepolishing at two planes was used as a sample for transparencymeasurement. Spectroscope U-3410 manufactured by HITACHI Co. Ltd. wasused and the measurement wavelength was set at 600 nm.

Degree of Crystallinity

All scattering intensity of X-ray was measurement with respect tocrystallized glass samples. From the results, the degree ofcrystallinity can be calculated by the following equation. X-raydiffraction apparatus MXP18A manufactured by Mac Science Co. Ltd. wasused as a X-ray diffraction apparatus.x=((1−(Σla/Σla100))×100x=(Σlc/Σlc100)×100

-   -   1a: Scattering intensity of an amorphous part of an unknown        material    -   1c: Scattering intensity of a crystalline part of an unknown        material    -   1a100: Scattering intensity of a material containing 100% of        amorphous.    -   1c100: Scattering intensity of a material containing 100% of        crystal        Measurement of Thermal Expansion Coefficient

Glass samples were cut off, and subjected to polishing process as toform a column shape of φ5 mm×20 mm, then they were used as samples forTMA measurement. TAS100 manufactured by Rigaku Co. Ltd. was used as ameasurement apparatus. As for the measurement conditions, the elevatingspeed of temperature was set as 4 K/min., the maximum temperature wasset as 350° C., and then the thermal expansion coefficients at from 100to 300° C. were measured.

As seen from the results shown in Tables 1 to 12, the crystallizedglasses of the present invention in Examples 1 to 42 had characteristicsof strength, such as the Young's modulus (equal to or higher than 140GPa), the specific modulus of elasticity (in the range of from 40 to 60MNm) and the like in a large value. Therefore, in the case of usingthese glasses as substrates for information recording medium such as amagnetic recording medium, even if these glass substrates are rotated ata high speed, they are not likely to exhibit warp or walking, and henceit is understood that they can meet the demand of further thinnersubstrates. In addition, when the liquid phase temperature was measuredwith respect to glasses before heat treatment in Example 1, Example 4and Example 10, the temperature was 1300° C., 1290° C. and 1270° C.,respectively, which satisfy the liquid phase temperature (for example,equal to or less than 1350° C.) required from the viewpoint of glassmelting and molding. In addition, when the average particle diameter ofthe crystal particle was measured with a Transmission ElectronMicroscopy (TEM) with respect to the crystallized glass in Examples 1 to42, the particles having the average particle diameter of from 20-30 nmto 100-150 nm were observed In addition, the polished planes subjectedto optical glass polishing for surface roughness measurement weresubjected to surface observation with an Atomic Force Microscopy (AFM)with respect to the crystallized glass in all Examples. From theresults, the surface roughness (Ra (JIS B0601)) of the crystallizedglasses other than in Examples 20, 21 and 24 was equal to or less than0.5 nm. The surface roughness (Ra (JIS B0601)) of these crystallizedglasses can be polished to equal to or less than 0.5 nm with, forexample, the polishing method for conventional optical glass usingabrasives such as synthesis diamond, silicon carbide, calcium oxide,iron oxide, cerium oxide. Therefore, substrates excellent in flatnesscan be obtained and it is useful as glass substrates for magneticrecording media for the purpose of smaller flying height. Thecrystallized glass of the present invention had the light transparencyequal to or higher than 50% at a wavelength of 600 nm in the case of thethickness of 1 mm and had transparency to some extent. Such transparencycan be an index showing whether desirable kinds of crystals and particlediameter of crystals are obtained. In the case of the crystallized glassof the present invention, said transparency can range, for example, from60 to 90%. For example, as the particle diameter of the crystal wassmaller, so said transparency becomes larger.

On the contrary, while the chemically reinforced glass substrate inComparative Example 1 shown in Table 13 was excellent in the surfacesmoothness and flatness, it was much inferior in characteristics ofstrength such as heat resistance and the Young's modulus than thecrystallized glass of the present invention. Accordingly, when magneticrecording media were produced, heat treatment to a magnetic layer forobtaining high coercive force could not be carried out sufficiently,thereby the magnetic recording media having high coercive force couldnot be obtained. In addition, the glass in Comparative Example 1contained large amount of alkali, so that the collosion of the magneticfilm with the substrate easily occurred, thereby it was afraid that themagnetic film was damaged.

In addition, the crystallized glass substrate in Comparative Example 2was inferior in the Young's modulus and smoothness than the glass of thepresent invention. In particular, because the smoothness of thesubstrate was deteriorated by the existence of large crystal particles,it was difficult to attempt high-density recording.

Production Method of Magnetic Disk

As exhibited in FIG. 1, the magnetic disk 1 of the present inventioncomprises a crystallized glass substrate 2 in said Example 1, on whichunevenness control layer 3, underlying layer 4, magnetic layer 5,protective layer 6 and lubricating layer 7 are provided in this order.

Each layer will be explained in detail. The substrate 2 was a diskhaving an outer circular periphery radius of 32.5 mm, inner circularperiphery radius of 10.0 mm and thickness of 0.43 mm, whose mainsurfaces were subjected to precision polishing so that they should havesurface roughness Ra of 4 Å and R_(max) of 40 Å.

The unevenness control layer is a thin AlN layer of 5-35% nitrogencontent having average roughness of 50 Å and surface roughness R_(max)of 150 Å.

The underlying layer is a thin layer of CrV composed of Cr: 83 at % andV: 17 at % having a thickness of about 600 Å.

The magnetic layer is a thin layer of CoPtCr composed of Co: 76 at %,Pt: 6.6 at %, Cr: 17.4 at % having a thickness of about 300 Å.

The protective layer is a carbon thin layer having a thickness of about100 Å.

The lubricating layer is a layer having a thickness of 8 Å, which wasformed by applying perfluoropolyether on the carbon protective layer byspin coating.

The method for producing magnetic disks will be explained hereinafter.

The crystallized glass of Example 1 was cut into a disk having an outercircular periphery radius of 32.5 mm, inner circular periphery radius of10.0 mm and thickness of 0.5 mm and the both main surfaces weresubjected to precision polishing so that they should have surfaceroughness Ra of 4 Å and R_(max) of 40 Å to afford a crystallized glasssubstrate for magnetic recording medium.

Subsequently, the above crystallized glass substrate was placed on asubstrate holder and transferred into a charging chamber of inlinesputtering apparatus. Then, the holder on which the crystallized glasssubstrate was placed was transferred to a first chamber where an Altarget was etched and sputtering was performed at a pressure of 4 mtorrand substrate temperature of 350° C. in an atmosphere of Ar+N₂ gas(N₂=4%) . As a result, an AlN thin layer having surface roughnessR_(max) of 150 Å and thickness of 50 Å (unevenness forming layer) wasprovided on the crystallized glass substrate.

The holder on which the crystallized glass substrate having the formedAlN layer was placed was then transferred into a second chamber providedwith a CrV target (Cr: 83 at %, V: 17 at %) and second chamber providedwith a CoPtCr target (Co: 76 at %, Pt: 6.6 at %, Cr: 17.4 at %)successively, and thin layers. were formed on the substrate. Sputteringwas performed at a pressure of 2 mtorr and substrate temperature of 350°C. in an Ar atmosphere, thereby a CrV underlying layer having athickness of about 600 Å and CoPtCr magnetic layer having a thickness ofabout 300 Å were formed.

The substrate having the formed unevenness control layer, underlyinglayer and magnetic layer was then transferred to a fourth chamberprovided with a heater for heat-treatment. The fourth chamber had aninner atmosphere of Ar gas (pressure: 2 mtorr) and the heat treatmentwas performed with changing the heat treatment temperature.

The substrate was then transferred into a fifth chamber provided with acarbon target, and a carbon protective layer having a thickness of about100 Å was formed under the same condition as used for forming of the CrVunderlying layer and the CoPtCr magnetic layer except that the layer wasformed in an atmosphere of Ar+H₂ gas (H₂=6%)

Finally, the substrate after forming the carbon protective layer wastaken out from the above inline sputtering apparatus, and a lubricatinglayer having a thickness of 8A was formed by applying perfluoropolyetheron the carbon protective layer by dipping.

The substrate for information recording medium of the present inventioncan be easily molded, and has the large Young's modulus equal to orhigher than 140 GPa, high heat resistance, excellent surfaceproductivity and surface smoothness (less than 10 Å of surface roughnessRa (JIS B0601) ) as well as it can be used as a substrate materialhaving large hardness and strength.

In addition, because the substrate composed of the crystallized glass ofthe present invention is excellent in heat resistance, heat treatmentrequired for improving characteristics of magnetic films can be usedwithout deformation of substrates. Because it is excellent in flatness,smaller flying height, that is, high-density recording, can beaccomplished. Because it has high Young's modulus, specific modulus ofelasticity and strength, it has advantages that thinner magnetic diskscan be obtained and the magnetic disks can rotate at a high speed whileavoiding the fracture of the magnetic disks.

In addition, the crystallized glass of the present invention can beproduced with relative stability, and production in the industrial scalecan be done easily, so that it can be greatly expected as substrateglasses for cheap magnetic recording media in the next generation.

1. An information recording medium comprising a recording layer on asubstrate, said substrate composed of crystallized glass comprisingSiO₂, Al₂O₃, MgO and alkali metal oxides and being substantially free ofZnO, wherein the crystallized glass comprises Al₂O₃ in an amount of 5 to25 mol % and alkali metal oxides in an amount equal to or less than 5mol %, main crystals contained in the crystallized glass consist ofenstatite and/or its solid solution, thermal expansion coefficient ofthe crystallized glass at 100 to 300° C. is in the range of from 65×10⁻⁷to 85×10⁻⁷/° C., and the substrate has a polished surface with a surfaceroughness Ra (JIS B0601) equal to or less than 1 nm.
 2. An informationrecording medium comprising a recording layer on a substrate, saidsubstrate composed of crystallized glass comprising SiO₂, Al₂O₃, MgO,Y₂O₃ and alkali metal oxides, wherein the crystallized glass comprisesAl₂O₃ in an amount of 5 to 25 mol %, alkali metal oxides in an amountequal to or less than 5 mol % and Y₂O₃ in an amount of 0.3 to 10 mol %,main crystals contained in the crystallized glass consist of enstatiteand/or its solid solution, and the substrate has a polished surface witha surface roughness Ra (JIS B0601) equal to or less than 1 nm.
 3. Aninformation recording medium comprising a recording layer on asubstrate, said substrate composed of crystallized glass comprisingSiO₂, Al₂O₃, MgO and alkali metal oxides, and being substantially freeof ZnO, wherein the crystallized glass comprises Al₂O₃ in an amount of 5to 25 mol %, alkali metal oxides in an amount equal to or less than 5mol %, wherein K₂O is present in an amount which ranges from 0.1 to 2mol %, main crystals contained in the crystallized glass consist ofenstatite and/or its solid solution, and the substrate has a polishedsurface with a surface roughness Ra (JIS B0601) equal to or less than 1nm.
 4. The recording medium according to claim 1, wherein thecrystallized glass further contains Y₂O₃ in an amount of 0.3 to 10 mol%.
 5. The recording medium according to claim 1, wherein Al₂O₃ ispresent in the substrate in an amount of 7 to 22 mol %.
 6. The recordingmedium according to claim 2, wherein Al₂O₃ is present in the substratein an amount of 7 to 22 mol %.
 7. The recording medium according toclaim 3, wherein Al₂O₃ is present in the substrate in an amount of 7 to22 mol %.
 8. The recording medium according to claim 1, wherein thethermal expansion coefficient of the crystallized glass at 100 to 300°C. is in the range of from 73×10⁻⁷ to 83×10⁻⁷/° C.
 9. The recordingmedium according to claim 1, wherein the crystallized glass comprisesAl₂O₃ in an amount of 7 to 22 mol % and the thermal expansioncoefficient of the crystallized glass at 100 to 300° C. is in the rangeof from 73×10⁻⁷ to 83×10⁻⁷/° C.
 10. The recording medium according toclaim 1, wherein the crystallized glass further contains K₂O in anamount of 0.1 to 2 mol %.
 11. The recording medium according to claim 2,wherein the crystallized glass further contains K₂O in an amount of 0.1to 2 mol %.
 12. The recording medium according to claim 1, wherein thecrystallized glass comprises: SiO₂ 35 to 65 mol % MgO 10 to 40 mol % andTiO₂ 5 to 15 mol %,

wherein the sum of SiO₂, Al₂O₃, MgO and TiO₂ is equal to or more than 93mol % and a molar ratio of Al₂O₃ to MgO is less than 0.5.
 13. Therecording medium according to claim 2, wherein the crystallized glasscomprises: SiO₂ 35 to 65 mol % MgO 10 to 40 mol % and TiO₂ 5 to 15 mol%,

wherein the sum of SiO₂, Al₂O₃, MgO and TiO₂ is equal to or more than 93mol % and a molar ratio of Al₂O₃ to MgO is less than 0.5.
 14. Therecording medium according to claim 3, wherein the crystallized glasscomprises: SiO₂ 35 to 65 mol % MgO 10 to 40 mol % and TiO₂ 5 to 15 mol%,

wherein the sum of SiO₂, Al₂O₃, MgO and TiO₂ is equal to or more than 93mol % and a molar ratio of Al₂O₃ to MgO is less than 0.5.
 15. Therecording medium according to claim 1, wherein the crystallized glasscomprises: SiO₂ 35 to 65 mol % Al₂O₃ 7 to 22 mol % MgO 10 to 40 mol %,and TiO₂ 5 to 15 mol %,

wherein the sum of SiO₂, Al₂O₃, MgO and TiO₂ is equal to or more than 93mol % and a molar ratio of Al₂O₃ to MgO is less than 0.5.
 16. Therecording medium according to claim 2, wherein the crystallized glasscomprises: SiO₂ 35 to 65 mol % Al₂O₃ 7 to 22 mol % MgO 10 to 40 mol %,and TiO₂ 5 to 15 mol %,

wherein the sum of SiO₂, Al₂O₃, MgO and TiO₂ is equal to or more than 93mol % and a molar ratio of Al₂O₃ to MgO is less than 0.5.
 17. Therecording medium according to claim 3, wherein the crystallized glasscomprises: SiO₂ 35 to 65 mol % Al₂O₃ 7 to 22 mol % MgO 10 to 40 mol %,and TiO₂ 5 to 15 mol %,

wherein the sum of SiO₂, Al₂O₃, MgO and TiO₂ is equal to or more than 93mol % and a molar ratio of Al₂O₃ to MgO is less than 0.5.
 18. Therecording medium according to claim 2, wherein the crystallized glass issubstantially free of ZnO.
 19. The recording medium according to claim1, wherein the substrate has a polished surface with a surface roughnessRa (JIS B0601) equal to or less than 0.5 nm.
 20. The recording mediumaccording to claim 2, wherein the substrate has a polished surface witha surface roughness Ra (JIS B0601) equal to or less than 0.5 nm.
 21. Therecording medium according to claim 3, wherein the substrate has apolished surface with a surface roughness Ra (JIS B0601) equal to orless than 0.5 nm.
 22. The recording medium according to claim 1, whereinlight transparency at 600 nm through the substrate with 1 mm thicknessis equal to or more than 10%.
 23. The recording medium according toclaim 2, wherein light transparency at 600 nm through the substrate with1 mm thickness is equal to or more than 10%.
 24. The recording mediumaccording to claim 3, wherein light transparency at 600 nm through thesubstrate with 1 mm thickness is equal to or more than 10%.
 25. Therecording medium according to claim 1, wherein the crystallizationdegree of the crystallized glass is equal to or more than 50 vol %, aswell as in the crystalline phase, the total content of enstatite and/orits solid solution ranges from 70 to 90 vol %, titanate is present in anamount which ranges from 10 to 30 vol %, and the sum of enstatite and/orits solid solution and titanate is equal to or more than 90 vol %. 26.The recording medium according to claim 2, wherein the crystallizationdegree of the crystallized glass is equal to or more than 50 vol %, aswell as in the crystalline phase, the total content of enstatite and/orits solid solution ranges from 70 to 90 vol %, titanate is present in anamount which ranges from 10 to 30 vol %, and the sum of enstatite and/orits solid solution and titanate is equal to or more than 90 vol %. 27.The recording medium according to claim 3, wherein the crystallizationdegree of the crystallized glass is equal to or more than 50 vol %, aswell as in the crystalline phase, the total content of enstatite and/orits solid solution ranges from 70 to 90 vol %, titanate is present in anamount which ranges from 10 to 30 vol %, and the sum of enstatite and/orits solid solution and titanate is equal to or more than 90 vol %. 28.The recording medium according to claim 1, wherein the crystallizedglass is substantially free of spinel as a crystal phase.
 29. Therecording medium according to claim 2, wherein the crystallized glass issubstantially free of spinel as a crystal phase.
 30. The recordingmedium according to claim 3, wherein the crystallized glass issubstantially free of spinel as a crystal phase.